For F10 M5, it claims to increase performance to 615 hp & 750Nm.
One good thing (if true) is that you can unplug it & go back to stock setting when going to BMW for service.

This product is basically a very crude way of increasing power. It's been very popular in the diesel tuning arena after the common rail diesel engines came along.

The power box simply alters the signals that some sensors (on a common rail diesel it's the fuel pressure sensor) give to the ECU. Fooling the ECU to believe that it's running a low fuel pressure and increasing fuel pressure to increase power.

Say that the (diesel) fuel pressure is supposed to be 1800bar under normal conditions. The fuel pressure sensor gives a certain value/feedback to the ECU when the pressure is 1800bar. By bypassing the signal via the power box and altering that signal the ECU believes the fuel pressure is just 1500bar and increases the signal to the fuel pump and gets the fuel pressure higher. This then leads to the ECU receiving the correct fuel pressure feedback for 1800bar, but the real fuel pressure is 2100bar...!!!

This is probably also the same method applied to the M5. Most likely both fuel and boost pressure signals are altered to fool the ECU into believing that it's running low boost and fuel pressure...

The power boxes are really nothing more than a added resistance in the circuit to some of the sensors...

This product is basically a very crude way of increasing power. It's been very popular in the diesel tuning arena after the common rail diesel engines came along.

The power box simply alters the signals that some sensors (on a common rail diesel it's the fuel pressure sensor) give to the ECU. Fooling the ECU to believe that it's running a low fuel pressure and increasing fuel pressure to increase power.

Say that the (diesel) fuel pressure is supposed to be 1800bar under normal conditions. The fuel pressure sensor gives a certain value/feedback to the ECU when the pressure is 1800bar. By bypassing the signal via the power box and altering that signal the ECU believes the fuel pressure is just 1500bar and increases the signal to the fuel pump and gets the fuel pressure higher. This then leads to the ECU receiving the correct fuel pressure feedback for 1800bar, but the real fuel pressure is 2100bar...!!!

This is probably also the same method applied to the M5. Most likely both fuel and boost pressure signals are altered to fool the ECU into believing that it's running low boost and fuel pressure...

The power boxes are really nothing more than a added resistance in the circuit to some of the sensors...

Yes, this should be how it works. Since it kinda of 'fools' the ECU, without actually altering the ECU settings, if you unplug it & go for service, in theory, BMW should not able to detect anything & thus no issue with warranty. Is that right ?

The other questions are :
- does it actually produce such performance gain ?
- any drawbacks for using this kind of chipbox ?

Yes, this should be how it works. Since it kinda of 'fools' the ECU, without actually altering the ECU settings, if you unplug it & go for service, in theory, BMW should not able to detect anything & thus no issue with warranty. Is that right ?

The other questions are :
- does it actually produce such performance gain ?
- any drawbacks for using this kind of chipbox ?

BMW might be able to detect unusual output signals from the ECU. Ie if the voltage to the fuel pump normally is 11V to maintain normal fuel pressure and on your car the log shows that it's repeatedly running at 12V there might be some issues with BMW (just a very crude illustration).

Performance gains are possible as long as boost and fuel is higher, but these powerboxes have often been over advertised.

The drawbacks are obviously that the ECU are still operating at it's std. maps and believes that the boost pressure still is std. The ECU maps are not altered to advance/retard ignition etc to the higher boost and fuel pressure. Meaning that the map is still for a 560HP engine setup with a given boost pressure and not remapped for higher boost and HP. This can lead to problems with lean/rich mixture, exhaust temperatures etc.

BMW might be able to detect unusual output signals from the ECU. Ie if the voltage to the fuel pump normally is 11V to maintain normal fuel pressure and on your car the log shows that it's repeatedly running at 12V there might be some issues with BMW (just a very crude illustration).

Performance gains are possible as long as boost and fuel is higher, but these powerboxes have often been over advertised.

The drawbacks are obviously that the ECU are still operating at it's std. maps and believes that the boost pressure still is std. The ECU maps are not altered to advance/retard ignition etc to the higher boost and fuel pressure. Meaning that the map is still for a 560HP engine setup with a given boost pressure and not remapped for higher boost and HP. This can lead to problems with lean/rich mixture, exhaust temperatures etc.

Ahh finally a voice of reason

You're correct BMW can detect changes in signals, temperatures, revs, altitude etc
and if they don't correlated in a reasonable way they likely will log an fault . Not all faults are stored in a visible fashion so BMW can also retrieve hidden faults the aftermarket code readers/erasers won't see. That's why some of the code readers/erasers will flood the ECU with false/benign codes (such as exhaust flap) to (hope) erase any other codes

In short if it would be that easy to come up with a simple piggy back tune (or any other) the bigger tuners would have done so already.

As now has been demonstrated over and over again, there's no magic 8 ball to tune the M5 and BMW will certainly make sure to keep tuners at bay and introduce new and improved measures. The honest tuners providing tunes today will tell you that they can modify the ECU/maps but it will also void your engine / drive train warranty.
It all depends how desperate you are and how much of a risk you're willing to take.

Won't work as your EWS will be miss-aligned (ie car won't start) and plenty of other things won't work and throw errors etc (mileage, modules, VIN etc)
Replacing the DME is not an exercise you want to try at home